It is common for a receiver in an elevator system to employ a voltage level converter for converting the voltage of a transmitted signal to a level suitable to circuitry connected to the receiver.
A large amount of noise is inherent in the transmission and reception of electric signals in an elevator system due to the use of many electric and electronic components which have a wide range of voltage and current requirements. To deal with noise caused by short circuits and ground loops in the wiring of the elevator system, it is common to use a voltage level converter which includes an optical coupler. In other words, an input voltage is converted to an optical signal when a current through an LED of a primary side of the optical coupler. The secondary side of the optical coupler, electrically insulated from the primary side and faulty wiring connected to it, receives optical signals and a voltage signal resembling the input voltage is reconstructed from it. The voltage across the LED is reduced by placing a power resistor in series with the optical coupler.
One way to deal with induced or capacitive coupled noise is to lower the input impedance of the voltage level converter by decreasing the resistor value, thereby increasing the current needed to turn on the LED of the optical coupler. This is a self-defeating solution to the noise problem because of the high power consumption of the voltage level converter caused by a high current needed for LED turn-on. Further, the expense of a power resistor capable of enduring high currents makes this voltage level converter unattractive from a cost standpoint. The voltage level converter is unattractive from a manufacturing standpoint because the size of such a power resistor makes the voltage level converter unsuitable for surface mounting.